Eighty-five p.c of the universe consists of darkish matter, however we don’t know what, precisely, it’s.
A brand new examine from the College of Michigan, Lawrence Berkeley Nationwide Laboratory (Berkeley Lab) and College of California, Berkeley has dominated out darkish matter being liable for mysterious electromagnetic alerts beforehand noticed from close by galaxies. Previous to this work there have been excessive hopes that these alerts would give physicists onerous proof to assist establish darkish matter.
Darkish matter can’t be noticed straight as a result of it doesn’t take up, replicate or emit gentle, however researchers comprehend it exists due to the impact it has on different matter. We want darkish matter to clarify gravitational forces that maintain galaxies collectively, for instance.
Physicists have urged darkish matter is a carefully associated cousin of the neutrino, referred to as the sterile neutrino. Neutrinos — subatomic particles with no cost and which hardly ever work together with matter — are launched throughout nuclear reactions going down contained in the solar. They’ve a tiny quantity of mass, however this mass isn’t defined by the Normal Mannequin of Particle Physics. Physicists recommend that the sterile neutrino, a hypothetical particle, may account for this mass and likewise be darkish matter.
Researchers ought to be capable to detect the sterile neutrino as a result of it’s unstable, says Ben Safdi, co-author and an assistant professor of physics at U-M. It decays into abnormal neutrinos and electromagnetic radiation. To detect darkish matter, then, physicists scan galaxies to hunt for this electromagnetic radiation within the type of X-ray emission.
In 2014, a seminal work found extra X-ray emission from close by galaxies and galaxy clusters. The emission seemed to be according to that which might come up from decaying sterile neutrino darkish matter, Safdi mentioned.
Now, a meta evaluation of uncooked information taken by the XMM-Newton house X-ray telescope of objects within the Milky Manner over a interval of 20 years has discovered no proof that the sterile neutrino is what includes darkish matter. The analysis group consists of U-M doctoral scholar Christopher Dessert and Nicholas Rodd, a physicist with the Berkley Lab principle group and the Berkley Heart for Theoretical Physics. Their outcomes are revealed within the journal Science.
“This 2014 paper and follow-up works confirmed the signal generated a significant amount of interest in the astrophysics and particle physics communities because of the possibility of knowing, for the first time, precisely what dark matter is at a microscopic level,” Safdi mentioned. “Our finding does not mean that the dark matter is not a sterile neutrino, but it means that — contrary to what was claimed in 2014 — there is no experimental evidence to-date that points towards its existence.”
Area-based X-ray telescopes such because the XMM-Newton telescope, level at dark-matter-rich environments to seek for this faint electromagnetic radiation within the type of X-ray alerts. The 2014 discovery named the X-ray emission the “3.5 keV line” — keV stands for kilo-electronvolts — due to the place the sign appeared on X-ray detectors.
The analysis group looked for this line in our personal Milky Manner utilizing 20 years of archival information taken by the XMM-Newton house X-ray telescope. Physicists know darkish matter collects round galaxies, so when earlier analyses checked out close by galaxies and galaxy clusters, every of these photographs would have captured some column of the Milky Manner darkish matter halo.
The group used these photographs to take a look at the “darkest” a part of the Milky Manner. This considerably improved the sensitivity of earlier analyses searching for sterile neutrino darkish matter, Safdi mentioned.
“Everywhere we look, there should be some flux of dark matter from the Milky Way halo,” mentioned the Berkeley Lab’s Rodd, due to our photo voltaic system’s location within the galaxy. “We exploited the fact that we live in a halo of dark matter” within the examine.
Christopher Dessert, a examine co-author who’s a physics researcher and Ph.D. scholar at U-M, mentioned galaxy clusters the place the 3.5 keV line have been noticed even have giant background alerts, which function noise in observations and may make it troublesome to pinpoint particular alerts that could be related to darkish matter.
“The reason why we’re looking through the galactic dark matter halo of our Milky Way galaxy is that the background is much lower,” Dessert mentioned.
For instance, XMM-Newton has taken photographs of remoted objects like particular person stars within the Milky Manner. The researchers took these photographs and masked the objects of authentic curiosity, leaving pristine and darkish environments by which to seek for the glow of darkish matter decay. Combining 20 years of such observations allowed for a probe of sterile neutrino darkish matter to unprecedented ranges.
If sterile neutrinos had been darkish matter, and if their decay led to an emission of the 3.5 keV line, Safdi and his fellow researchers ought to have noticed that line of their evaluation. However they discovered no proof for sterile neutrino darkish matter.
“While this work does, unfortunately, throw cold water on what looked like what might have been the first evidence for the microscopic nature of dark matter, it does open up a whole new approach to looking for dark matter which could lead to a discovery in the near future,” Safdi mentioned.
Reference: “The dark matter interpretation of the 3.5-keV line is inconsistent with blank-sky observations” by C. Dessert; B.R. Safdi at College of Michigan, Ann Arbor in Ann Arbor, MI; N.L. Rodd at College of California, Berkeley in Berkeley, CA; N.L. Rodd at Lawrence Berkeley Nationwide Laboratory in Berkeley, CA., 26 March 2020, Science.
Researchers within the examine had been supported by the U.S. Division of Vitality’s Early Profession Analysis Program, Leinweber Heart for Theoretical Physics at U-M and Miller Institute for Primary Analysis in Science at UC Berkeley. The analysis was supported by Superior Analysis Computing at U-M.